PolyADP-ribosylation is a unique posttranslational modification of proteins, involved in various cellular functions including stability of chromatin. PolyADP-ribosylation modifies acceptor proteins with a large negatively charged poly(ADP-ribose) (PAR) to greatly change the structure and function of the acceptor proteins. In addition various specific motifs of proteins were recently found to interact non-covalently with PAR thereby changing the spaciotemporal activity of protein-protein interaction in cells. However, the structure of PAR to which specific protein motifs should bind is not fully characterized. The present work will review the structure, physicochemical properties and quantification of PAR in vivo, with special reference to PAR binding protein modules.